Organic electroluminescent devices using an organic substance are promising in applications as a cheap large-area full-color display device having a solid light-emitting solid-state device and a light source array for writing, and a number of developments have been carried out. In general, an organic electroluminescent device is constructed of a light emitting layer and a pair of counter electrodes disposed sandwiching the light emitting layer therebetween. When an electric field is applied between the both electrodes, an electron is injected from the cathode, and a hole is injected from the anode. The light emission is a phenomenon in which the electron and the hole are re-coupled in the light emitting layer, and energy is released as light during a time when the energy level is returned to a valence band from a conductor.
However, in the case of such an organic electroluminescent device, there is a serious problem that the luminous efficiency is very low as compared with inorganic LED devices and fluorescent tubes.
Almost all of organic electroluminescent devices which are currently proposed are ones utilizing fluorescent light emission obtained by a singlet exciton of an organic light emitting material. In a simple mechanism of the quantum chemistry, in the exciton state, a ratio of the singlet exciton from which fluorescent light emission is obtainable to the triplet exciton from which phosphorescent light emission is obtainable is 1/3. So far as the fluorescent light emission is utilized, only 25% of the exciton can be effectively applied so that the luminous efficiency is low.
On the other hand, if phosphorescence obtainable from the triplet exciton can be utilized, the luminous efficiency should be able to be enhanced. Actually, in recent years, organic electroluminescent devices utilizing phosphorescence with a phenylpyridine complex of iridium have been reported, and it is reported that such organic electroluminescent devices exhibit the luminous efficiency of 2 to 3 times as compared with the conventional organic electroluminescent devices utilizing fluorescence (for example, see U.S. Pat. No. 6,303,238, Applied Physics Letter, 1999, Vol. 75, page 4, and Japanese Journal of Applied Physics, 1999, Vol. 38, pages L1502 to L1504).
Most of phosphorescence organic electroluminescent devices have a device construction of anode/hole transport layer/light emitting layer/block layer/electron transport layer/cathode. In the light emitting layer, a host material having a function to undergo energy transfer of triplet exciton energy into a light emitting material and a phosphorescent material are mainly used. In general, carbazole compounds such as CBP (4,4′-N,N″-di-carbazolebiphenyl) are frequently used as the host material.
In order to efficiently transfer triplet exciton energy from the host material into the phosphorescent material, it is necessary that a lowest energy level T1 of the triplet excited state of the host material is larger than that of the light emitting material. In a green light emitting material or a red light emitting material, even if CBP is used as the host material, since the T1 of CBP becomes sufficiently larger than that of the light emitting material, a high luminous efficiency is obtained.
However, in the case of a blue light emitting material, since the T1 of the light emitting material becomes larger than that of CBP as the host material, it becomes difficult to efficiently transfer the triplet exciton energy, resulting in a serious problem that the luminous efficiency is largely lowered.
As a host material having a large T1, which is useful as the blue light emitting material, there is reported CDBP (4,4′-bis(9-carbazolyl)-2,2′-dimethyl-biphenyl) resulting from incorporation of a methyl group into the biphenyl of CBP (for example, see Applied Physics Letter, 2003, Vol. 83, page 569). However, its luminous efficiency is still low, and its durability is also problematic.
On the other hand, in the light emitting layer, it is possible to undergo light emission only with a light emitting material without containing a host material. However, quenching is generated due to a mutual action among the light emitting materials, resulting in a problem that both luminance and luminous efficiency become low.
For the purpose of solving this problem, there is proposed a method in which an electrically inactive inorganic dielectric material and a light emitting material are made co-present in the light emitting layer (for example, see JP-A-2001-196178). However, in order to subject the inorganic dielectric material and an organic material which is the light emitting material to film formation jointly, thereby forming the light emitting layer, the light emitting material which can be used is restricted.
Also, there is disclosed a method in which an electrically inactive polymer binder is used in the light emitting layer (for example, see JP-A-2002-180040). However, since the polymer is used, the production process of an organic electroluminescent device is limited to a wet film formation process (see JP-A-2002-180040).